This invention relates to a high-density wired circuit board having a required wiring pattern by using insulated wires.
Printed circuit boards are mainly produced by a subtractive process wherein a wiring pattern is formed by using a copper-clad laminate and dissolving and removing unnecessary portions by chemicals. On the other hand, there also exists an additive process wherein a wiring pattern is formed by electroless plating on a laminate having no copper foil thereon. According to these processes, since the wiring is formed on a plane, crossed wiring cannot be formed on the same plane. The crossed wiring is obtained by using different planes, perforating through-holes therethrough, and metallizing inner walls of the through-holes by electroless plating or the like to form interlayer connection. In order to improve disadvantages of the prior art processes and to provide a high-density wiring board suitable for small-scale production, there are proposed wiring boards obtained by forming an adhesive layer on an insulating substrate and holding insulated wires on the adhesive layer (hereinafter referred to as "wired circuit board") in, for example, U.S. Pat. Nos. 3,674,914 and 3,674,602. The wired circuit boards are produced, as disclosed in these U.S. Patents, by laminating or coating a thermosetting adhesive which is thermoplastic at the time of wiring (by providing wires on the adhesive and at the same time tacking thereto) on an insulating substrate made from thermosetting resin laminates or the like, wiring insulated wires coated with a heat resistant resin such as polyimide resin, or the like by using a numerical control wiring machine, fixing the wired wires by pressing or the like, perforating through-holes at the ends of wires while crossing the wires, exposing cut ends of wires to inner walls of the through-holes and forming a metallic layer at the inner walls of the through-holes by electroless plating to connect cut ends of the wires. According to the thus produced wired circuit boards, since insulated coated wires are used, insulating properties of the wires are high even if the wires are crossed or wired closely in parallel and are thus suitable for high-density wiring.
However, the wired circuit boards have a problem of cross-talk noise between insulated wires closely positioned in parallel when 3 or more wires are provided in a pitch of 2.54 mm between through-holes to make the pitch between insulated wires 0.3 mm or less. Therefore, in the case of a circuit used for high-speed signals, there is a limit for wiring two insulated wires between a pitch of 2.54 mm between through-holes. Such a circuit is insufficient in wiring capacity required for smaller sized devices and has a high production cost due to an increase in the required number of wiring layers.